Large-field-of-view waveguide supporting red, green, and blue in one plate

ABSTRACT

An optical device for combining RGB optical signals in a single waveguide. The device includes a plurality of DOEs. A first DOE is configured to receive an optical signal at input propagation angles and to diffract the optical signal based on spectrum such that predominately one spectrum of light is diffracted in a first direction and path and predominately a second spectrum of light is diffracted in a second different direction and path. The first DOE is configured to diffract light into a second DOE. The second DOE is configured to diffract light into a third DOE. The third DOE is configured to diffract light into an eye box keeping output propagation angles substantially parallel to the input propagation angles. A summation of grating vectors for each of the paths is substantially equal to zero.

BACKGROUND Background and Relevant Art

Recently, there has been a resurgence in the interest in virtual reality(VR) and augmented reality (AR) devices and other such near eye devices.These devices typically include a video transmitter of some sort, suchas a light engine, and optics couple to the video transmitter configuredto transmit images to the eyes of the user using the devices. Inparticular, a user will wear a headset or similar device that includes avideo transmitter optically coupled to one or more waveguides where thewaveguides are configured to optically couple images out to a user.

One problem that has needed to addressing by manufacturers of suchdevices is a problem related to limited Field of View (FoV). In thecontexts illustrated herein, the FoV is the number of degrees of visualhigh angle assuming a fixed eye position. Horizontally, the FoV for ahuman is around 135°. However, often virtual reality and augmentedreality devices will have a much lower FoV available. The lower the FoVavailable from the device, the less realistic the experience with thedevice.

Technologies have been implemented which attempt to widen the FoV. Onesuch technology is the use of diffraction gratings which spread thelight by wavelength to increase the FoV. That is, a diffraction gratingis dispersive, which means that it creates diffraction orders such thatthe colors of all non-zero orders propagate in different directions.While this behavior is highly beneficial, e.g., in spectroscopicapplications, in AR/VR devices based on diffractive waveguides it isunwanted, since carrying and expanding the image content in thewaveguide requires three (or in some cases two) separate waveguidesunless the FoV is very small.

Having multiple waveguides greatly complicates the manufacturingprocess. Not only one must manufacture several waveguides butmanufacturing tolerances become much tighter. In addition, one mustaccurately put the multiple plates in a grating stack, which addsadditional manufacturing steps which require high accuracy, andincreased cost.

The subject matter claimed herein is not limited to embodiments thatsolve any disadvantages or that operate only in environments such asthose described above. Rather, this background is only provided toillustrate one exemplary technology area where some embodimentsdescribed herein may be practiced.

BRIEF SUMMARY

One embodiment illustrated herein includes an optical device forcombining RGB optical signals in a single waveguide. The device includesa plurality of DOEs. The device includes a first DOE configured toreceive an optical signal at input propagation angles and to diffractthe optical signal based on spectrum such that predominately onespectrum of light is diffracted in a first direction and predominately asecond spectrum of light is diffracted in a second different directionsuch that different portions of optical signal take different paths,including at least two different paths. The device includes a secondDOE. The first DOE is configured to diffract light into the second DOE.The device includes a third DOE. The second DOE is further configured todiffract light into the third DOE. The second and third DOE areconfigured to cause expansions that are substantially non-parallel. Thethird DOE is configured to diffract light into an eye box keeping outputpropagation angles within some predetermined threshold of the inputpropagation angles. The plurality of DOEs are associated with gratingvectors. A summation of grating vectors for each of the paths in the atleast two different paths is substantially equal to zero.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

Additional features and advantages will be set forth in the descriptionwhich follows, and in part will be obvious from the description, or maybe learned by the practice of the teachings herein. Features andadvantages of the invention may be realized and obtained by means of theinstruments and combinations particularly pointed out in the appendedclaims. Features of the present invention will become more fullyapparent from the following description and appended claims, or may belearned by the practice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features can be obtained, a more particular descriptionof the subject matter briefly described above will be rendered byreference to specific embodiments which are illustrated in the appendeddrawings. Understanding that these drawings depict only typicalembodiments and are not therefore to be considered to be limiting inscope, embodiments will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 illustrates an example of a near-eye display device;

FIG. 2 illustrates various display elements;

FIG. 3 illustrates a waveguide;

FIG. 4A illustrates a wave vector space representation;

FIG. 4B illustrates a wave vector space representation;

FIG. 4C illustrates a wave vector space representation;

FIG. 5 illustrates an output waveguide;

FIG. 6A illustrates a waveguide with odd-order expansion;

FIG. 6B illustrates a waveguide with odd-order expansion;

FIG. 7 illustrates a waveguide with even-order expansion; and

FIG. 8 illustrates a method of combining RGB optical signals in a singlewaveguide.

DETAILED DESCRIPTION

Some embodiments illustrated herein may include or may be used toimplement a diffractive waveguide based AR/VR device that 1) carriesvirtual content from a light engine to the front of a user's eye and 2)expands the pupil, thus enlarging the eye box. In particular, someembodiments, can support a large FoV (e.g., 45×30°) in a singlewaveguide plate for multiple different wavelengths. For example,embodiments may be configured to support red, green, and blue (RGB)wavelengths with a large FoV, in a single waveguide. Thus, embodimentsmay carry large FoV RGB content through a single waveguide. This can beaccomplished in some embodiments as now illustrated.

Some embodiments include various diffractive optical elements (DOEs) ina waveguide to accomplish the functionality described herein. In oneexample embodiment an incoupling grating (referred to herein as DOE1)diffracts light into two or more directions such that one spectrum ofwavelengths, e.g., the red light, is diffracted primarily in a differentdirection(s) than another spectrum of wavelengths e.g., blue light. Inthe illustrated example, green light is split between these directions.This can be accomplished for example, by using a single-sided crossedgrating (doubly-periodic grating) or by using linear gratings on the twosurfaces of the waveguide. While the examples illustrated herein referto the two (or more) paths through the waveguide as the red path and theblue path, it should be appreciated that other color spectrum paths maybe implemented. Further, it should be appreciated that in practice, partof red light (or other colors) naturally goes through the blue path (orother colors), and vice versa.

As will be illustrated in further detail below, there are differentexpansion gratings (illustrated herein as DOE2) for the blue path andthe red path. Both have at least one distinct wing of DOE2 but may alsohave more. The number of wings for DOE2 can also be unequal for thesetwo paths.

The out-coupling grating (illustrated herein as DOE3) has two differentperiods and orientations (or more if multiple colors are handledseparately) for the red and blue. Again, this can be done by crossedgrating on one side of a grating or “crossed” linear gratings, one oneach of the different surfaces of the waveguide.

Note that in components where light reaches DOE3 by multiple possiblepaths, each path obeys a zero summation rule separately such that thesummations of vectors for each path sums to approximately zero, asillustrated in more detail below.

Thus, in general, embodiments may split the FoV of different colors intotwo (or more) paths, carry the partial FoVs to DOE3 while expanding thepupil by pupil replication, and at DOE3 recombining the differentcontributions of each color.

Additional details are now illustrated.

FIG. 1 shows an example of a near-eye display device in whichembodiments can be practiced. The near-eye display device 100 may be avirtual reality (VR) and/or augmented reality (AR) device that canprovide a VR or AR experience with the user. In a VR experience,essentially the entire visual experience is provided by the VR device'slight engine. In an AR experience, the light engine is used to transmitimages onto a transparent protective visor. In this way, the visualexperience includes elements provided by the light engine of the VRdevice as well as objects that can be seen visually by the user throughthe transparent protective visor. In the examples illustrated herein,the near-eye display device 100 is designed for AR visualization, but VRdevices can be implemented using the principles illustrated.

In the illustrated embodiment, the near-eye display device 100 includesa chassis 101, a transparent protective visor 102 mounted to the chassis101, and left and right side arms 104 mounted to the chassis 101. Thevisor 102 forms a protective enclosure for various display elementsshown in FIG. 2.

A display assembly 200 (see FIG. 2) that can generate images for AR/VRvisualization is also mounted to the chassis 101 and enclosed within theprotective visor 102. The visor assembly 102 and/or chassis 101 may alsohouse electronics to control the functionality of the display assembly200 and other functions of the near-eye display device 100. The near-eyedisplay device 100 further includes an adjustable headband 105 attachedto the chassis 101, by which the near-eye display device 100 can be wornon a user's head.

FIG. 2 shows a side view of display components that may be containedwithin the visor 102 of the near-eye display device 100, in someembodiments of the invention. During operation of the near-eye displaydevice 100, the display components are positioned relative to the user'sleft eye 206 _(L) or right eye 206 _(R). The display components aremounted to the interior surface of the chassis 101. The chassis 101 isshown in cross-section in FIG. 2.

In an AR application, the display components are designed to overlaythree-dimensional images on the user's view of a real-world environmentviewable through the transparent protective visor 102, e.g., byprojecting light into the user's eyes. Accordingly, the displaycomponents include a display module 204 that houses a light engineincluding components such as: one or more light sources (e.g., one ormore light emitting diodes (LEDs)); one or more microdisplay imagers,such as liquid crystal on silicon (LCOS), liquid crystal display (LCD),digital micromirror device (DMD); and one or more lenses, beam splittersand/or waveguides. The microdisplay imager(s) (not shown) within thedisplay module 204 may be connected via a flexible circuit connector 205to a printed circuit board 208 that has image generation/controlelectronics mounted on it.

The display components further include a transparent waveguide carrier201 to which the display module 204 is mounted, and one or more outputwaveguides 202 on the user's side of the waveguide carrier 201, for eachof the left eye and right eye of the user. Note that, ideally,embodiments are able to use a single waveguide to implement thefunctionality described herein. The waveguide carrier 201 has a centralnose bridge portion 210, from which its left and right waveguidemounting surfaces extend. Waveguides 202 are implemented on each of theleft and right waveguide mounting surfaces of the waveguide carrier 201,to project light emitted from the display module and representing imagesinto the left eye 206 _(L) and right eye 206 _(R), respectively, of theuser. The display assembly 200 can be mounted to the chassis 101 througha center tab 207 located at the top of the waveguide carrier 201 overthe central nose bridge section 210.

The near-eye display device can provide light representing an image toan optical receptor (e.g., an eye) of a user. The user may be, e.g., ahuman, an animal or a machine.

FIG. 3 shows an example of an output waveguide that can be mounted onthe waveguide carrier 201 to convey light to one eye of the user. Asimilar waveguide can be designed for the other eye (or eyes), forexample, as a (horizontal) mirror image of the waveguide shown in FIG.3. The waveguide 310 is transparent (although diffractive) and, as canbe seen from FIG. 2, would normally be disposed directly in front of theeye of the user during operation of the near-eye display device, e.g.,as one of the waveguides 202 in FIG. 2. The waveguide 310 is, therefore,shown from the user's perspective during operation of the near-eyedisplay device 100.

The waveguide 310 includes a single input port 311, which is a DOEindicated as DOE1 (also called in-coupling element). The input port 311may be formed from, for example, a surface diffraction grating, volumediffraction grating, or a reflective component.

In the example illustrated herein, the input port 311 is configured todiffract input light into two or more spectra (with some leakage of theother specta) and to diffract those two or more spectra in differentdirections. This causes the different spectra to take different paths onthe transmission channel 312 illustrated in FIG. 3.

This is illustrated in one detailed example illustrated in FIG. 4A. FIG.4A illustrates a wave vector space representation. FIG. 4A shows atransverse wave vector space representation of light waves beingdiffracted by DOE1, input port 311 in the waveguide 310. The inner solidcircle 401 represents the border of total internal refraction (TIR)condition. The outer solid circle 402 represents the border ofevanescent waves.

Therefore, any light waves in the doughnut shaped portion betweenconcentric circles 401 and 402 propagate in the waveguide 310 by totalinternal reflection (TIR). Any light waves in the inner circle 401 arewaves propagate in the waveguide and then exit into the air. In otherwords, those light waves propagate in the waveguide and then exit fromthe waveguide. Any light waves outside of the outer circle 402 areevanescent waves that are not coupled into the waveguide.

FIG. 4A shows two grating vectors for DOE1 and the FOVs diffracted byDOE1 of the waveguide 310. In particular, FIG. 4A shows a DOE1 bluepath, −1 order, a DOE1 red path, −1 order, a DOE1 red path, +1 order anda DOE1 blue path, +1 order.

In some embodiments, DOE1 may include a linear grating with a firstgrating orientation and period on the front surface of the grating and asecond grating orientation and period on the back of the grating. Thefirst grating can diffract one spectrum of light, and the second gratingcan diffract a second spectrum of light. Alternatively, DOE1 may includea cross grating on one side of a waveguide. The grating vectors of thecross grating may have different orientations and lengths, and they maybe non-orthogonal to each other.

Referring once again to FIG. 3, the waveguide 310 includes atransmission channel 312. The transmission channel includes a DOE,referred to herein as DOE2. Note that DOE2 has several different wings,including DOE2 top left, DOE2 top right, DOE2 bottom left and DOE2bottom right. As noted previously, DOE2 comprises a number of expansiongratings. The functionality of DOE2, will be explained in more detailbelow in conjunction with the description of FIGS. 5 through 7.

However, reference is now made to FIG. 4B which illustrates DOE2 gratingvectors and FOVs diffracted by DOE2.

Note that the various wings of DOE2 may be implemented on a grating witha first wing on the front of the grating, and a second wing on the backof the grating. In some embodiments, these first and second wings canoverlap. In some embodiments, DOE2 may be a linear grating with firstand second wings on the front of the waveguide. In some embodiments,DOE2 may be a linear grating with first and second wings on the back ofthe waveguide.

Referring once again to FIG. 3, The waveguide 310 further includes asingle output port 313, which is a DOE indicated as DOE3 (also calledout-coupling element).

Referring now to FIG. 4C, DOE3 grating vectors and FOVs diffracted byDOE3 are shown.

During operation, the display module 204 (see FIG. 2) outputs lightrepresenting an image for an eye from its output port into the inputport 311 of the waveguide 310.

The transmission channel 312 conveys light from the input port 311 tothe output port 313 and may be, for example, a surface diffractiongrating, polarization grating, volume diffraction grating, or areflective component. The transmission channel 312 may be designed toaccomplish this by use of total internal reflection (TIR). Lightrepresenting the image is then projected from the output port 313 to theuser's eye.

Thus, in general, embodiments may split the FoV of different colors intotwo (or more) paths, carry the partial FoVs to DOE3 while expanding thepupil by pupil replication, and at DOE3 recombining the differentcontributions of each color. Two or more paths may be identical in someparts of the path.

The grating vectors of DOEs 1, 2 and 3 satisfy D₁+D₂+D₂=0.

Specifically, the two grating vectors of DOE1 (for the +1 order) aredenoted by D1r and D1b for “red” and “blue” paths as illustrated in theFigures above.

DOE2 grating vectors are denoted by D2tr, D2br, D2tl, D2bl fortop-right, bottom-right, top-left, bottom-left, respectively

Grating vectors of DOE3 are denoted by D3b and D3r

Then the path equations are:D1r+D2bl+D3r=0−D1r+D2tr+D3r=0D1b+D2br+D3b=0−D1b+D2tl+D3b=0

FIG. 4A-4Cc presents an example of a k-vector map enabling this type ofsolution. Note that in FIG. 3, a part of both red and blue FoV appear tobe leaky but this is not necessary the case in all embodiments.

The waveguide 310 may include multiple diffraction optical elements(DOEs), in order to control the directions of the light propagating inthe near-eye display device via multiple occurrences of opticaldiffraction. The DOEs may be, for example, surface diffraction gratingsor volume diffraction gratings. Various components of the waveguide 310can be designed to contain one or more of the DOEs.

For example, the waveguide 310 may include three DOEs. The input port311 of the waveguide 310 is a DOE1 for coupling light into the waveguide310 and controlling the direction of light path after the light reachesthe input port 311.

The transmission channel 312 of the waveguide 310 is a DOE2 forcontrolling the direction of light path in the transmission channel 312and ensuring the light propagating inside of the transmission channel312 through total internal reflection (TIR). Further, DOE2 is configuredhomogenize light signals in a horizontal direction

The output port 313 is a DOE3 for controlling the direction of the lightpath after the light exits the output port 313. DOE3 configured todiffract light into an eye box keeping output propagation angles withinsome predetermined threshold of the input propagation angle

The propagation directions of the expanded light waves are substantiallyparallel to each other (within some predetermined threshold). Theexpanded light waves are spaced or distributed along the particulardirection.

In other words, the expanded light waves are translated along theparticular direction (or coordinate axis) in an output waveguide beforeexiting the output waveguide. Each of the expanded light waves has arelatively narrow range of propagation angles or FoV. Each expandedlight wave has a “propagation vector” representing the averagepropagation direction of the light wave and denoting a center axis ofthe prorogation energy of the expanded light wave. Translation of alight wave means shifting the corresponding propagation vector of thelight wave along a particular direction (or coordinate axis) that is notparallel to the propagation vector itself.

Thus, the light waves exiting the output waveguide have the samedirection as (i.e., are substantially parallel, within some threshold,to) the light waves entering the output waveguide for light of any givenwavelength, to have the light waves follow the desired path to theoptical receptor of a user. This condition is called achromatic imaging.

The following illustrates details with respect to expanding light and isdirected to a single light path. However, it should be appreciated thatthe concepts illustrated can be applied to the different paths describedabove, such that expansion and the summation rules apply for eachdistinct path of light.

The waveguide including three DOEs can expand the light waves in twodimensions. The expansion process is also referred to as exit pupilexpansion. FIG. 5 shows an example of an output waveguide that expandsthe exit pupil of a near-eye display device. The waveguide 510 includesthree DOEs 515, 520 and 525 to expand the exit pupil. The DOEs 515, 520and 525 are successive in a common light path. The DOEs 515, 520 and 525can be, e.g., arranged on a planar substrate.

The imager 505 (e.g., an LCOS device) outputs a light wave 550 that isincident upon the first DOE 515 in a Z direction. The DOE 515 directsthe light wave 552 toward the second DOE 520. As shown in FIG. 5, theDOE 520 expands the light wave 554 in a first dimension (X dimension).As shown in FIG. 5, during the expansion, each propagation vector of theexpanded light waves 554 is shifted along the X coordinate axis suchthat the expanded light waves are spaced or distributed in the Xdimension.

The DOE 520 further redirects the expanded light wave 554 to a third DOE525. The third DOE 525 further expands the light wave 554 in a seconddimension (Y dimension), and redirects the expanded light wave 556outward in the Z direction.

Thus, the waveguide 510 receives the input light wave 550 incident inthe Z direction, expands the light wave in both X and Y dimensions, andredirects the expanded light waves in the same Z-direction. In otherwords, the waveguide 510 expanded light distribution in two dimensionswhile maintains the direction of the light wave. Thus, the waveguide 510can be referred to as a beam-expanding device or an exit pupil expander.

The waveguide, as a beam-expanding device, can expand light waves in,e.g., an odd-order expansion process or an even-order process. FIG. 6Ashows an output waveguide conducting an odd-order expansion. Thewaveguide 600 includes DOEs 615, 620 and 625.

Each of the DOEs 615, 620 and 625 has a diffraction grating. Adiffraction grating is an optical component with a periodic structure,which splits and diffracts an incident light beam into several beamstravelling in different directions. The periodic structure can includelinear grooves arranged in a periodic pattern. The distance betweennearby grooves is called grating period d.

The diffraction grating has a property of grating vector D (alsoreferred to as diffraction pattern vector). The grating vector Drepresents the direction and spacing of the grating pattern (alsoreferred to as periodic diffraction pattern). The length of a gratingvector is D=2π/d. The direction of the grating vector D is perpendicularto (“normal to” or “orthogonal to”) center axes of the periodic lineargrooves, where the center axes are perpendicular to the cross sectionsof the periodic linear grooves.

Light is incident upon the waveguide 600 in a Z direction, which isperpendicular to the X and Y directions. The first DOE 615 couples lightfrom an imager (not shown) into the waveguide 600. The second DOE 620expands the light in the X direction. The third DOE 625 further expandsthe light in the Y direction and couples the expanded light out from thewaveguide 600 in the same Z direction.

As shown in FIG. 6A, the second DOE 620 receives the light wave from thefirst DOE 615 at a left edge (as the reader views the figure) of the DOE620. The light wave is reflected by the grating pattern in the DOE 620for one or more times before the light wave exits the DOE 620 at abottom edge of the DOE 620. Because the odd-order expansion enables thesecond DOE 620 to receive the light wave at a side edge, a waveguide ofan odd-order expansion configuration usually occupies less space than awaveguide of an even-order expansion configuration (which is discussedlater).

During the odd-order expansion process, the second DOE 620 reflects(i.e., changes the direction of) the light for an odd number of timesbefore redirecting the light into the third DOE 625. Over the process ofmultiple reflections between 0 and +1 diffraction orders, a greaterportion of the light energy is converted to +1 order, which isredirected toward the third DOE 625.

FIG. 6B shows the wave vectors of light propagating in the waveguide andgrating vectors of DOEs of the waveguide. The incident light has a pairof transverse wave vector components k_(x0) and k_(y0). The magnitude ofthe wave vector is the wave number k=2π/λ, where λ is the wavelength ofthe light. The wave number of the incident light in the air is denotedas k₀. The wave number of the light propagating in the waveguide isdenoted as k=k₀*n, where n is the refractive index of the waveguidematerial.

The grating vectors of the DOE1, 2, and 3 (616, 620 and 625 in FIG. 6B)are denoted as D_(j)=(D_(xj), D_(yj)). The DOE 615 with a wave vector of(D_(x1), D_(y1)) redirects the incident light (k_(x0), k_(y0)) towardthe second DOE 620. Therefore, (k_(x1), k_(y1))=(k_(x0)+D_(x1),k_(y0)+D_(y1)).

The DOE 620 with a wave vector of (D_(x2), D_(y2)) receives light(k_(x1), k_(y1)) and redirects the light (k_(x1), k_(y1)) toward thethird DOE 625. Therefore, (k_(x2), k_(y2))=(k_(x1)+D_(x2),k_(y1)+D_(y2))=(k_(x0)+D_(x1)+D_(x2)+D_(x3),k_(y0)+D_(y1)+D_(y2)+D_(x3)).

The DOE 625 with a wave vector of (D_(x3), D_(y3)) receives light(k_(x2), k_(y2)) and couples the light (k_(x2), k_(y2)) out in a Zdirection. Therefore, (k_(x3), k_(y3))=(k_(x2)+D_(x3),k_(y2)+D_(y3))=(k_(x0)+D_(x1)+D_(x2)+D_(x3),k_(y0)+D_(y1)+D_(y2)+D_(x3)).

The waveguide 600 satisfies the achromatic imaging condition, whichmeans that when the light waves with different wavelengths are expandedby the waveguide 600 and exit the waveguide 600, the exit directions ofthe light waves are the same as the input directions in which the lightwaves enter the waveguide 600. In other words, the incident light wavenumber (k_(x0), k_(y0)) matches the out-coupled light wave number(k_(x3), k_(y3)): (k_(x0), k_(y0))=(k_(x3), k_(y3)). Therefore, thegrating vectors of the waveguide 600 satisfyD_(x1)+D_(x2)+D_(x3)=D_(y1)+D_(y2)+D_(x3))=0. Alternatively, in a vectorform, a vector summation of the grating vectors equals zero:

D₁+D₂+D₂=0 (also referred to as the “summation rule”).

Note that the grating vectors D₁, D₂, D₂ depend on grating periods butdo not depend on wavelengths of the light waves. Therefore, once thegrating vectors satisfy the summation rule, the achromatic imagingcondition is satisfied for light waves with any wavelengths (hence theterm “achromatic imaging”).

To satisfy the achromatic imaging condition, it is not necessary torestrict the diffraction gratings of first DOE 615 and the DOE 625 tohave the same grating period. The summation rule relaxes the designlimitations of those diffraction gratings. The relaxed designlimitations enable a waveguide 600 to have a larger FoV.

Furthermore, the waveguide 600 keeps the light diffracted by DOEs 615and 620 inside the waveguide 600. Thus, the light propagating inside ofthe waveguide 600 is not evanescent and satisfies condition of totalinternal reflection (TIR). In other words, light diffracted by DOE 615satisfies the TIR condition inside of the waveguide: k_(x1) ²+k_(y1)²>k₀ ². Light diffracted by DOE 615 is not evanescent: k_(x1) ²+k_(y1)²<k². Light diffracted by DOE 620 also satisfies the TIR conditioninside of the waveguide: k_(x2) ²+k_(y2) ²>k₀ ². Light diffracted by DOE620 is not evanescent: k_(x2) ²+k_(y2) ²<k².

Although FIGS. 6A, 6B and 6C shows a waveguide including three DOEs, awaveguide according to the disclosed technology can have any arbitrarynumber of DOEs. For example, if a waveguide includes N number of DOEs,the condition of achromatic imaging is D_(x1)+D_(x2)+D_(x3)+ . . .+D_(xN)=D_(y1)+D_(y2)+D_(x3)+ . . . +D_(yN)=0. Alternatively, in avector form: D₁+D₂+D₂+ . . . +D_(N)=0. The DOEs also satisfy theconditions for TIR and non-evanescence.

In some embodiments, the achromatic imaging condition can be expressedas a weighted vector summation of the grating vectors: mD₁+nD₂+lD₃=0,where the values m, n, and l in the addends are integer weight valuesthat represent diffraction orders to which the periodic diffractionpatterns are designed to concentrate light energy. In some embodiments,the integer weight values can be 0, negative, or positive.

Furthermore, the waveguide, as a beam-expanding device, can expand lightwaves in an even-order expansion process as well. FIG. 7 shows an outputwaveguide conducting an even-order expansion. The waveguide 700 includesDOEs 715, 720 and 725.

Light is incident upon the waveguide 700 in a Z direction, which isperpendicular to the X and Y directions. The first DOE 715 couples lightinto the waveguide 700, and redirects the light wave into the second DOE720 at a top edge of DOE 720. The second DOE 720 expands the light inthe X direction. The third DOE 725 further expands the light in the Ydirection and couples the expanded light out from the waveguide 700 inthe same Z direction.

As shown in FIG. 7, the second DOE 720 receives the light wave from thefirst DOE 715 at the top edge of the DOE 520. Note that in the odd-orderexpansion illustrated in FIG. 6A, the second DOE 520 receives the lightwave at the left side edge. The choice of either odd-order expansion oreven-order expansion depends on various design factors for thewaveguide. Typically, a waveguide of odd-order expansion configurationtends to be smaller. An even-order expansion configuration, on the otherhand, enables supplying the light wave at the top edge of the secondDOE, which may be advantageous when there is a limitation on the widthof the waveguide.

The light wave is reflected by the grating pattern in the DOE 720multiple times before the light wave exits the DOE 720 at a bottom edgeof the DOE 720. During the even-order expansion process, the second DOE720 reflects the light an even number of times (including zero time)before redirecting the light into the third DOE 525. Similar to theodd-order expansion, over the process of multiple reflections between 0and +1 orders, more of the light energy is converted to +1 order, whichis redirected toward the third DOE 725.

As shown in FIG. 7, the second DOE 720 expands the light wave in the Xdirection. However, the second DOE 720 maintains the direction of itsoutput light as the same of the direction of its input light. In otherwords, in the even-order expansion, the wave vectors of light wavesbefore and after second DOE 720 are identical. Thus, the grating vectorfor the diffraction grating of the second DOE 720 does not imposelimitation to diffraction vectors of other DOEs in the waveguide 700.

In the even-order expansion, the first DOE 715 can have, e.g., lineardiffraction gratings on two sides of the DOE 715 (also referred to as“dual-sided linear grating”). The first diffraction grating on a firstside (e.g., top side) of DOE 715 has a grating vector ofD_(1a)=(D_(x1a), D_(y1a)). The second diffraction grating on a secondside (e.g., bottom side) of DOE 715 has a grating vector ofD_(1b)=(D_(x1b), D_(y1b)). The diffraction grating of the third DOE 725has a grating vector of D₃=(D_(x3), D_(y3))

The waveguide 700 satisfies the achromatic imaging condition, whichmeans the incident light (k_(x0), k_(y0)) matches the out-coupled light(k_(x3), k_(y3)). The achromatic imaging condition is satisfied, ifmD_(1a)+nD_(1b)=±D₃, wherein m and n are integer order numbers.

In some embodiments, the achromatic imaging condition can be expressedas a weighted vector summation of the grating vectors:mD_(1a)+nD_(1b)+lD₃=0, where the values m, n and l in the addends areinteger weight values that represent diffraction orders to which theperiodic diffraction patterns are designed to concentrate light energy(also referred to as “weighted summation rule”). In some embodiments,the integer weight values can be 0, −1 or +1. Higher diffraction orders,corresponding to integer numbers whose absolute values are larger than1, are usually suppressed by the grating patterns.

In some embodiments, m=1 and n=0, or m=0 and n=1. Thus, the first DOE715 has one diffraction grating with a wave vector D₁=±D₃. In otherwords, if the first DOE 715 and the third DOE 725 have the same lengthfor the grating vectors (or the same grating period), the achromaticimaging condition is satisfied.

The design limitation of the grating vectors can be further relaxed,because the grating periods for first DOE 715 and third DOE 725 do notneed to be equal. In some embodiments, m=1 and n=1, which means thefirst diffraction grating of the first DOE 715 reflects the light waveto +1 diffraction order, and then the second diffraction grating of thefirst DOE 715 reflects the light wave again to +1 diffraction order.Diffraction orders higher than the +1 diffraction order usually are lessefficient and can create ghost image effects. Thus, when m=1 and n=1, avector sum of the grating vectors of the diffraction gratings of thefirst DOE 715 either equals the grating vector of the third DOE 725, oris the exact opposite to the grating vector of the third DOE 725:D_(1a)+D_(1b)=±D₃. Particularly, in case of −D₃, the first and seconddiffraction gratings of the first DOE 715 and the diffraction grating ofthe third DOE 725 satisfy the summation rule: D_(1a)+D_(1b)+D₃=0.

Besides dual-sided linear grating, the first DOE 715 can have, e.g.,crossed diffraction gratings on two sides of the DOE 715 (also referredto as “dual-sided crossed grating”). Thus, the first DOE 715 effectivelyhave four diffraction gratings with four grating vectors. On a firstside (e.g., top side) of the first DOE 715, there are two diffractiongratings that are crossed to each other and have grating vectors ofD_(1a)=(D_(x1a), D_(y1a)) and D_(1b)=(D_(x1b), D_(y1b)). In other words,the grating pattern is periodic in two directions on the first side. Ona second side (e.g., bottom side) of the first DOE 715, there are twodiffraction gratings that are crossed to each other and have gratingvectors of D_(1c)=(D_(x1c), D_(y1c)) and D_(1d)=(D_(x1d), D_(y1d)).

The waveguide 700 satisfies the achromatic imaging condition, whichmeans the incident light matches the out-coupled light. The achromaticimaging condition is satisfied, if mD_(1a)+nD_(1b)+oD_(1a)+pD_(1b)=±D₃,wherein m, n, o, and p are integer order numbers.

Therefore, the weighted vector summation rule can be used to design DOEsof output waveguides. The diffraction gratings of DOEs follow thesummation rule or the weighted summation rule, and therefore satisfiesthe achromatic imaging order. The summation rule or the weightedsummation rule enables relaxed degrees of freedom for designing theconfiguration of the output waveguides with various properties of DOEs.

The following discussion now refers to a number of methods and methodacts that may be performed. Although the method acts may be discussed ina certain order or illustrated in a flow chart as occurring in aparticular order, no particular ordering is required unless specificallystated, or required because an act is dependent on another act beingcompleted prior to the act being performed.

Referring now to FIG. 8, a method 800 is illustrated. The method 800includes acts for combining RGB optical signals in a single waveguide.The waveguide includes a plurality of DOEs. The method incudes directingan optical signal at a first DOE at input propagation angles (act 802).

The method 800 further includes at the first DOE, diffracting theoptical signal based on spectrum such that predominately one spectrum oflight is diffracted in a first direction and predominately a secondspectrum of light is diffracted in a second different direction suchthat different portions of optical signal take different paths,including at least two different paths (act 804).

The method 800 further includes at the first DOE, diffracting thedifferent portions into a second DOE (act 806).

The method 800 further includes at the second DOE, diffracting thedifferent portions into a third DOE (808).

The method 800 further includes at the second DOE and the third DOEexpanding the optical signal in a substantially non-parallel fashion;Expansions at DOE2 and DOE3 are substantially non-parallel. For example,embodiments may expand the pupil at DOE2 essentially in the verticaldirection, and then at DOE3 in the horizontal direction during theoutcoupling process (act 810).

The method 800 further includes at the third DOE, diffracting thedifferent portions into an eye box keeping output propagation angleswithin some predetermined threshold of the input propagation angles.That is, an attempt is made to keep the output propagation anglessubstantially parallel to the input propagation angles to preventdistortion and/or other side-effects (act 812).

The plurality of DOEs are associated with grating vectors. The acts ofmethod 800 are performed such that a summation of grating vectors foreach of the paths in the at least two different paths is substantiallyequal to zero (act 814).

Note that being ‘substantially equal to zero’ is dependent on thedisplay resolution of a device. In particular, the summation issubstantially equal to zero so long as some predefined resolution ismaintained. In some embodiments, this may mean that the outputresolution of an outgoing optical signal must be the same as the inputresolution of an incoming optical signal.

The method 800 may be practiced where diffracting the optical signalbased on spectrum such that predominately one spectrum of light isdiffracted in a first direction and predominately a second spectrum oflight is diffracted in a second different direction such that differentportions of optical signal take different paths, including at least twodifferent paths is performed by the first DOE having a linear gratingassociated with a first grating vector on the front of the grating and asecond grating vector on the back of the waveguide.

The method 800 may be practiced where diffracting the different portionsinto an eye box keeping output propagation angles within somepredetermined threshold of the input propagation angles is performed bythe third DOE being a linear grating with a first grating vector on thefront of the waveguide and a second grating vector on the back of thewaveguide.

The method 800 may be practiced where expanding the optical signal in asubstantially non-parallel fashion and diffracting the differentportions into a third DOE is performed by the second DOE having a lineargrating with a first wing on the front and a second wing on the back ofthe waveguide, wherein the first wing and the second wings overlap.

The method 800 may be practiced where expanding the optical signal in asubstantially non-parallel fashion and diffracting the differentportions into a third DOE is performed by the second DOE having a lineargrating with a first wing and a second wing on the front of thewaveguide.

The method 800 may be practiced where expanding the optical signal in asubstantially non-parallel fashion and diffracting the differentportions into a third DOE is performed by the second DOE having a lineargrating with a first wing and a second wing on the back of thewaveguide.

The method 800 may be practiced where diffracting the optical signalbased on spectrum such that predominately one spectrum of light isdiffracted in a first direction and predominately a second spectrum oflight is diffracted in a second different direction such that differentportions of optical signal take different paths, including at least twodifferent paths is performed by the first DOE having a cross gratingassociated with two distinct grating vectors.

The method 800 may be practiced where expanding the optical signal in asubstantially non-parallel fashion and diffracting the differentportions into a third DOE is performed by the second DOE having a crossgrating associated with two distinct grating vectors.

The method 800 may be practiced where diffracting the different portionsinto an eye box keeping output propagation angles within somepredetermined threshold of the input propagation angles is performed bythe third DOE having a cross grating associated with two distinctgrating vectors.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or characteristics. The described embodimentsare to be considered in all respects only as illustrative and notrestrictive. The scope of the invention is, therefore, indicated by theappended claims rather than by the foregoing description. All changeswhich come within the meaning and range of equivalency of the claims areto be embraced within their scope.

What is claimed is:
 1. An optical device for combining RGB opticalsignals in a single waveguide, the device comprising a plurality of DOEsincluding: a first DOE comprising a first linear grating having a firstgrating period and a second linear grating having a second gratingperiod, the first DOE being configured to receive an optical signal atinput propagation angles and to diffract the optical signal based onspectrum such that a +1 diffraction order of a predominately firstspectrum of light is diffracted by the first linear grating in a firstdirection and a −1 diffraction order of the predominantly first spectrumof light is diffracted in a second direction by the first linear gratingand a +1 diffraction order of a predominately a second spectrum of lightis diffracted in a third direction by the second linear grating and a −1diffraction order of the predominantly second spectrum of light isdiffracted in a fourth direction by the second linear grating, such thatdifferent portions of optical signal take different paths, including atleast four different paths; a second DOE comprising at least four wingswith different grating orientations, each grating orientation beingoriented to diffract light toward a third DOE, wherein the first DOE isconfigured to diffract the optical signal diffracted in the firstdirection toward a first wing, the optical signal diffracted in thesecond direction toward a second wing, the optical signal diffracted inthe third direction toward a third wing, and the optical signaldiffracted in the fourth direction toward a fourth wing; the third DOEconfigured to diffract light into an eye box keeping output propagationangles within some predetermined threshold of the input propagationangles; wherein the second and third DOE are configured to causeexpansions that are substantially non-parallel; and wherein theplurality of DOEs are associated with grating vectors and wherein asummation of grating vectors for each of the paths in the at least twodifferent paths is substantially equal to zero.
 2. The optical device ofclaim 1, wherein at least one of the first DOE and the third DOEcomprise a linear grating associated with a first grating vector on thefront of the waveguide and a second grating vector on a back of thewaveguide.
 3. The optical device of claim 1, wherein the second DOEcomprises a linear grating with a first wing on the front and a secondwing on a back of the waveguide, wherein the first wing and the secondwing overlap.
 4. The optical device of claim 1, wherein at least one ofthe at least four wings of the second DOE comprises a linear grating. 5.The optical device of claim 1, wherein the first wing and the secondwing of the second DOE comprise linear gratings on a back of thewaveguide.
 6. The optical device of claim 1, wherein the second DOEcomprises a cross grating associated with two distinct grating vectors.7. The optical device of claim 1, wherein the third DOE comprises across grating associated with two distinct grating vectors.
 8. A methodof combining RGB optical signals in a single waveguide, the waveguidecomprising a plurality of DOEs, the method comprising: directing anoptical signal at a first DOE at input propagation angles, wherein thefirst DOE comprises a first linear grating having a first grating periodand a second linear grating having a second grating period; at the firstDOE, diffracting the optical signal based on spectrum such that a +1diffraction order of a predominately first spectrum of light isdiffracted by the first linear grating in a first direction and a −1diffraction order of the predominantly first spectrum of the light isdiffracted in a second direction by the first linear grating and a +1diffraction order of a predominately second spectrum of light isdiffracted in a third direction by the second linear grating and a −1diffraction order of the predominantly second spectrum of light isdiffracted in a fourth direction by the second linear grating, such thatdifferent portions of optical signal take different paths, including atleast four different paths; at the first DOE, diffracting the differentportions into a second DOE, wherein the second DOE comprises at leastfour wings with different grating orientations, each grating orientationbeing oriented to diffract light toward a third DOE, and wherein thefirst DOE diffracts light in the first direction toward a first wing,light in the second direction toward a second wing, light in the thirddirection toward a third wing, and light in the fourth direction towardthe third wing; at the second DOE, diffracting the different portionsinto the third DOE; at the second DOE and the third DOE expanding theoptical signal in a substantially non-parallel fashion; at the thirdDOE, diffracting the different portions into an eye box keeping outputpropagation angles within some predetermined threshold of the inputpropagation angles; and wherein the plurality of DOEs are associatedwith grating vectors and wherein the acts are performed such that asummation of grating vectors for each of the paths in the at least twodifferent paths is substantially equal to zero.
 9. The method of claim8, wherein diffracting the different portions into an eye box keepingoutput propagation angles within some predetermined threshold of theinput propagation angles is performed by the third DOE being a lineargrating with a first grating vector on the front of the waveguide and asecond grating vector on a back of the waveguide.
 10. The method ofclaim 8, wherein expanding the optical signal in a substantiallynon-parallel fashion and diffracting the different portions into a thirdDOE is performed by at least two of the at least four wings of thesecond DOE having linear gratings disposed on opposite sides of thewaveguide such that the linear gratings overlap.
 11. The method of claim8, wherein expanding the optical signal in a substantially non-parallelfashion and diffracting the different portions into a third DOE isperformed by the first wing and the second wing of the second DOE havinglinear gratings on the front of the waveguide.
 12. The method of claim8, wherein expanding the optical signal in a substantially non-parallelfashion and diffracting the different portions into a third DOE isperformed by the first wing and the second wing of the second DOE havinglinear gratings on a back of the waveguide.
 13. The method of claim 8,wherein diffracting the optical signal based on spectrum such that a +1diffraction order of a predominately first spectrum of light isdiffracted by the first linear grating in a first direction and a −1diffraction order of the predominantly first spectrum of the light isdiffracted in a second direction by the first linear grating and a +1diffraction order of a predominately second spectrum of light isdiffracted in a third direction by the second linear grating and a −1diffraction order of the predominantly second spectrum of light isdiffracted in a fourth direction by the second linear grating, such thatdifferent portions of optical signal take different paths, including atleast four different paths is performed by the first DOE having a crossgrating associated with two distinct grating vectors.
 14. The method ofclaim 8, wherein expanding the optical signal in a substantiallynon-parallel fashion and diffracting the different portions into a thirdDOE is performed by the second DOE having a cross grating associatedwith two distinct grating vectors.
 15. The method of claim 8, whereindiffracting the different portions into an eye box keeping outputpropagation angles within some predetermined threshold of the inputpropagation angles is performed by the third DOE having a cross gratingassociated with two distinct grating vectors.
 16. A near eye opticaldevice, comprising: a light engine; a waveguide coupled to the lightengine; wherein the waveguide comprises: a first DOE comprising a firstlinear grating having a first grating period and a second linear gratinghaving a second grating period, the first DOE being configured toreceive an optical signal from the light engine at input propagationangles and to diffract the optical signal based on spectrum such that a+1 diffraction order of a predominately first spectrum of light isdiffracted by the first linear grating in a first direction and a −1diffraction order of the predominantly first spectrum of light isdiffracted in a second direction by the first linear grating and a +1diffraction order of a predominately second spectrum of light isdiffracted in a third direction by the second linear grating and a −1diffraction order of the predominantly second spectrum of light isdiffracted in a fourth direction by the second linear grating, such thatdifferent portions of optical signal take different paths, including atleast four different paths; a second DOE comprising at least four wingswith different grating orientations, each grating orientation beingoriented to diffract light toward a third DOE, wherein the first DOE isconfigured to diffract the optical signal diffracted in the firstdirection toward a first wing, the optical signal diffracted in thesecond direction toward a second wing, the optical signal diffracted inthe third direction toward a third wing, and the optical signaldiffracted in the fourth direction toward a fourth wing; the third DOEconfigured to diffract light into an eye box keeping output propagationangles within some predetermined threshold of the input propagationangles; wherein the second and third DOEs are configured to causeexpansions that are substantially non-parallel; and wherein the first,second, and third plurality of DOEs are associated with grating vectorsand wherein a summation of grating vectors for each of the paths in theat least two different paths is substantially equal to zero.
 17. Thenear-eye optical device of claim 16, wherein at least some gratings ofthe first DOE, the second DOE, or the third DOE are positioned on a backof the waveguide.
 18. The near-eye optical device of claim 16, whereinat least some gratings of the first DOE, the second DOE, or the thirdDOE comprise a cross grating associated with two distinct gratingvectors.